Make native.bridge property overridable

When a device uses native bridge, it may be included in the system
image, so overriding by vendor/build.prop is not an option.
Example of overriding can be seen here: ag/15881132

Test: build cf_x86_64_phone-userdebug with native-bridge and launch, observe
1. ro.dalvik.vm.native.bridge=native_bridge.so moved from
vendor/build.prop to system/build.prop
2. the prop is correctly set at runtime
3. translated apps work
Bug: 197153442

Change-Id: I6cd566dd4e0fac181e309cb7f282d086de09075b